In resonant converters, a switching circuit converts a DC voltage into a square wave voltage and provides it to a resonant circuit. As shown in FIG. 1, a pulse frequency modulation (PFM) method is generally adopted, which generates a clock signal CLK with a duty cycle of 50% through an oscillator to control the switching circuit, and adjusts the frequency of the clock signal CLK based on a feedback signal ISET. A change in the frequency of the clock signal CLK will alter the frequency of the aforementioned square wave voltage, thereby causing a change in the gain of the resonant circuit and ultimately achieving adjustment of the output of the resonant converter.
However, the existing pulse frequency modulation method has a narrow bandwidth and cannot provide a timely and effective response during load transient, which results in a large overshoot or undershoot on the output.